The present invention relates to the field of fiber and fiber jacket retention devices and, more particularly, to an improved bushing for securing the position of the protective jacket of a jacketed optical fiber with respect to an optical component in an optical amplifying repeater.
Optical amplifiers comprise vital components in today""s optical submarine transmission systems. An optical amplifier is an optical device in which a section of rare earth doped optical fiber is pumped with light at the wavelength of the rare earth dopant, thereby causing population inversion of the dopant. The population inversion causes a signal to propagate at a signal wavelength along the fiber.
Erbium (Er) (atomic number 68) is particularly suited for use as a dopant in an optical amplifier. In particular, when erbium is excited it emits light at 1.54 xcexcm, which coincidentally is a low-loss wavelength for silica optical fibers. Thus, a signal may be amplified through an erbium-doped fiber amplifier (EDFA) by the process of xe2x80x9cstimulated emission.xe2x80x9d Specifically, when energy in the form of photons, such as from a laser diode, is pumped into the erbium atoms it causes the outer electrons of the erbium to jump to an excited state. When the erbium atoms subsequently are allowed to relax to a stable state, they release a 1.54 xcexcm photon.
In an optical submarine transmission system, the optical signal that is being transmitted through the submarine optical fiber cable becomes attenuated over the
In an optical submarine transmission system, the optical signal that is being transmitted through the submarine optical fiber cable becomes attenuated over the length of the transmission system, which may stretch thousands of miles. To compensate for this signal attenuation, optical repeaters are strategically positioned along the cable length. In the past, these optical repeaters were xe2x80x9cregenerativexe2x80x9d repeaters in which the optical signal being transmitted was first converted into an electrical signal, subjected to given processing (such as waveform shaping), and then converted back into an optical signal for further transmission through the cable. Today, however, more and more submarine optical fiber cable systems rely on xe2x80x9coptical amplifying repeatersxe2x80x9d (e.g., EDFAs) to compensate for signal attenuation and to boost signal strength.
In a typical EDFA, the optical fiber carrying the optical signal enters the EDFA and its signal is coupled through an optical coupler to the pumping light generated by one or more laser diodes (usually pumping at 980 nm or 1480 nm). Upon leaving the optical coupler, this coupled signal is carried by a short section of optical fiber that is ultimately spliced to a much longer section of erbium-doped optical fiber. Within the erbium-doped fiber, the erbium atoms of the doped fiber absorb the pumping light, thereby leading to population inversion and signal amplification. After the desired gain in signal strength has been achieved (which depends, in part, on the length of the section of erbium-doped fiber used in the EDFA), the pumping light is decoupled from the signal through a second optical coupler, thereby leaving only the amplified signal to continue through the optical fiber. The optical fiber carrying the amplified optical signal then exits the EDFA.
Bare optical fibers, however, are easily damaged by scratches or loads. Consequently, optical fibers are typically protected by one or more protective xe2x80x9cjackets.xe2x80x9d In a typical xe2x80x9cloose tubexe2x80x9d fiber construction, a 10 mil optical fiber (i.e., an optical fiber with a diameter of approximately 0.01 inches (or xe2x80x9c10 milxe2x80x9d)) passes through a 40 mil xe2x80x9cinner jacket,xe2x80x9d which is usually made of polyvinyl chloride (PVC). This inner jacket is then surrounded by an aramid strength member (e.g., Kevlar(copyright)) and a 60 mil xe2x80x9couter jacketxe2x80x9d (also made of PVC). The optical fiber, however, is not secured to either jacket. Instead, the fiber is free to xe2x80x9cfloatxe2x80x9d within the inner jacket and move longitudinally with respect to the jackets. Indeed, in submarine cable systems, the length of optical fiber within a jacketed optical fiber cable is usually longer than the length of the jacket itself, thereby helping alleviate any loads on the optical fiber that may be caused by thermal expansion or contraction or by other sources.
The length of the erbium-doped optical fiber used in an EDFA can easily exceed 20 meters. Because it is very difficult to store this length of optical fiber loose within an EDFA (especially within an EDFA used in submarine cable systems, where space is at a premium), most erbium-doped optical fiber used in an optical amplifying repeater is stored inside an EDFA on a spool in an erbium-doped fiber module (EDFM) that has been designed specifically for storing large quantities of erbium-doped fiber. One way that it is possible to store such a large quantity of fiber in a small component such as an EDFM is because the erbium-doped fiber within the EDFM is stored on the fiber spool xe2x80x9cjacketlessxe2x80x9dxe2x80x94i.e., after it has been stripped of its protective jackets-and thus has a much smaller diameter than regular xe2x80x9cjacketedxe2x80x9d optical fiber.
Outside of the EDFM, however, it is desirable that the optical fiber remain jacketed so that the fragile optical fiber itself is protected from damage. Indeed, because any damage to the fiber (even if only microscopic) may adversely affect the reliability of the EDFA (and, as a result, the reliability of the entire submarine optical fiber cable system), great efforts are normally taken to protect the optical fiber from damage.
Thus, to accomplish these dual goals of protecting the erbium-doped fiber from damage outside of the EDFM and facilitating the maximum amount of storage within the EDFM, it is preferable that the fiber be jacketed both when it enters and when it exits the EDFM, but jacketless within the EDFM itself. As discussed above, however, the optical fiber is free to xe2x80x9cfloatxe2x80x9d within its protective jackets and, conversely, the protective jackets are capable of moving longitudinally with respect to the optical fiber. Thus, the protective jackets of a jacketed optical fiber are preferably secured to the EDFM so that they do not move. If, for example, a protective jacket was allowed to move away from the EDFM, it might expose the optical fiber outside of the EDFM (and possibly allow the optical fiber to be damaged). Likewise, if a protective jacket was allowed to move freely into the EDFM, it may impinge the spool of xe2x80x9cjacketlessxe2x80x9d optical fiber stored therein and possibly lead to signal loss or to fiber damage.
Consequently, it is advisable that the protective jackets of the optical fiber be coupled to the external structure of the EDFM both when the optical fiber enters and leaves the EDFM. Coupling the jackets to the EDFM will prevent the jackets from moving with respect to the EDFM and thus will reduce the likelihood that the fragile optical fiber will be exposed, damaged, or impinged, even though the optical fiber itself will still be able to xe2x80x9cfloatxe2x80x9d freely with respect to its protective jackets and the EDFM.
One way of coupling a protective jacket to an EDFM is to wrap a bulking agent around the jacket and then wedge the wrapped jacket into an opening in the EDFM. In theory, this arrangement allows the jacket section of doped fiber to be secured to the EDFM.
Alternatively, one may use a rubber plug or xe2x80x9cbushing.xe2x80x9d In general, bushings are designed in such a way that the jacketed fiber could be pressed into the bushing and the bushing then placed into an opening in the EDFM. In the past, an element of the EDFM (e.g., a lid) would be closed, thereby applying a load onto the bushing itself and causing the bushing to compress and to xe2x80x9csqueezexe2x80x9d the protective jackets of the optical fiber. In this manner, the jackets would be secured to the EDFM, but because the jackets are generally incompressible, the optical fiber itself would still be free to float through the jackets and would not be coupled to the exterior structure of the EDFM.
Using a separate element of the EDFM to generate a load on the bushing, however, is not necessarily practical given the design of today""s EDFM trays. Moreover, using the lid of the EDFM to generate a load on the bushing is not advisable because when the lid is removed, the optical fiber stored on the spool within the EDFM could become dislocated. Instead, it would be desirable to have a bushing designed in such a way that the requisite xe2x80x9csqueezingxe2x80x9d load needed to secure the position of the optical fiber""s protective jacket with respect to the EDFM (or any optical component) could be generated merely by wedging the bushing into an opening in the optical component.
In light of the above, an improved bushing for securing the position of the protective jacket of a jacketed optical fiber with respect to an optical component (specifically, an EDFM) is provided. The improved bushing includes an elastomeric structure having a first end, a second end, and a jacketed fiber receiving surface. First and second flanges for engaging corresponding cavities within the optical component are attached to opposite sides of the elastomeric structure. The elastomeric structure further defines a cylindrical internal pathway for receiving a jacketed optical fiber, which includes the protective jacket(s) to be secured. The internal pathway runs approximately along the longitudinal centerline of the elastomeric structure from the first end to the second end and has a diameter approximately equal to the diameter of the jacketed optical fiber. The elastomeric structure also defines a channel from the internal pathway to the jacketed fiber receiving surface and which extends along the entire length of the internal pathway. Further, the elastomeric structure has an expanded state, an undeformed state, and a compressed state, and when the elastomeric structure is in its expanded state, the channel""s width is approximately the diameter of the internal pathway; when the elastomeric structure is in its undeformed state, the channel""s width is less than the diameter of the internal pathway; and when the elastomeric structure is in its compressed state, the channel""s width is less than it is when the elastomeric structure is in its undeformed state. A notch may also be defined in the jacketed fiber receiving surface for guiding the jacketed optical fiber into the channel.